For conventional antennas used either in detecting the electric fields of incoming radiation or in generating an electric field to emit outgoing waves, the design is dominated by the need to provide dimensions of the order of the wavelength of the detected or emitted radiation.
The sole known exception to this rule in general use is the well-known whip antenna. Another exception is my previous invention described and claimed in U.S. Pat. No. 3,914,766, entitled "Pulsating Plasma Device." This antenna is governed by the natural resonant frequency of the plasma. The present invention shows how a short antenna can be constructed using an electrostatic capacitor such as a split-cylindrical capacitor, as the radiating member of a resonant circuit. Since capacitors may be constructed in a wide variety of geometric shapes, requiring only a gap between two conductors which are electrically insulated from each other, a similar variety of antennas may be constructed by using suitable electrical components to provide resonance in the total electric circuit which includes the antenna as one element.
It is known from the extensive literature that the design problem for a particular use of the whip antenna is to obtain the proper value of Q over the frequency range of intended use. For the whip antenna the Q is approximately inversely proportional to the cube of the product of the antenna length and the frequency of use.
Contrast that situation with the present invention where it has been found that the Q of the split cylindrical antenna is proportional to the product of the frequency and the radius of the cylinder divided by the square of the length. Obviously the Q of the antenna will increase with frequency, all else being fixed, and therefore if desired the bandwidth can be much greater than that of a whip antenna designed for the same Q and central frequency.
The geometry of this antenna gives it a major advantage over the whip antenna. Because of its short vertical length it can be more easily and conveniently mounted on a vehicle. Its ability to detect horizontally polarized radiation when its major axis is vertical is another advantage when mounted on a vehicle. Since two major factors affect the polarization of incoming radiation, (the polarization of the transmitter and the reflection from the nearby ground), in many circumstances, the horizontally polarized radiation received at the vehicle would provide a much greater signal than the vertically polarized component detected by the usual whip antenna.
As used herein, a short length antenna is defined as one which has a length equal to or less than one quarter of a wavelength (.lambda./4) of its resonant frequency. Usually such short antennas typically exhibited a high Q or a rather sharp tuning peak.
Prior to the present invention, effective high frequency antennas, such as towers or beam antennas, were too large for house or apartment use. Many amateur radio operators resorted to apartment radiators, outdoor clothes lines, long wires stretched between poles or trees, house gutters and downspouts, flagpoles, and even bedsprings as compromise full size antennas.
For relatively small conventional indoor antennas (such as a dipole), small size, a lossy RF environment, and dielectric losses from wood, plaster, and masonry are serious handicaps to effective reception and transmission.
In portable and mobile applications, high frequency antennas usually require a loading coil to tune out capacitive reactance. Such coils introduce a coil loss which increases as the required resonating inductance gets larger and the radiation resistance decreases, i.e., at lower frequencies.
Sichak, in U.S. Pat. No. 2,633,532, describes a helically-slotted cylindrical antenna for transmitting high frequency energy. The pitch of the antenna slot is equal to an integral number of wavelengths of the frequency being radiated. Thus, Sichak is an example of a relatively long antenna.
Smith, in U.S. Pat. No. 2,812,514, describes a helically-slotted antenna having both right and left hand slots for radiating circularly polarized electromagnetic waves which have both vertical and horizontal components.
Carter, in U.S. Pat. No. 2,359,620, describes a short wave antenna for use over a wide band of frequency ranges. The antenna comprises a pair of parallel conducting discs facing each other and having axially aligned centers. The discs are energized at the center of their opposing faces and are designed so that their size is a fraction of the desired wavelength.
German Auslegeschrift No. 1,241,875 describes a high frequency antenna comprising a hollow metallic cylinder divided by two continuous, opposing parallel slits which divide the cylinder into two half cylinders. The half cylinders are insulated from each other. The diameter of the cylinder is approximately .lambda./3 and the width of a slit is approximately .lambda./10. The length of the cylinder is about 10 .lambda.. Thus, the antenna of German No. 1,241,875 is well over an order of magnitude larger than a short antenna as defined herein. The direction of transmission is along the axis of the cylinder.
While various antenna configurations utilizing some generally cylindrical conductors are described, for example, in U.S. Pat. Nos. 2,238,770, 2,633,532, 2,747,182, and 2,812,514, no antenna is known wherein the length of the antenna is less than .lambda./4 and wherein the Q of the antenna can be readily engineered by varying the radius of an axis of revolution of a pair of capacitor plates and a gap space between the plates.
In addition, while various slotted antenna and plate antenna configurations are described (for example in U.S. Pat. Nos. 2,359,620, 2,436,408, and 2,625,654), the prior art fails to provide a compact versatile antenna suitable for use in such wide ranging applications as FM receivers, high frequency HAM radio, and mobile radios.
Thus, a need exists for a small and efficient broad band antenna that can be used with both portable and mobile operations for receiving and transmitting polarized radiation.